The speed of a rotating machine is typically sensed by stationary sensors, such as magnetic speed pickups, mounted around the shaft and sensing the frequency of passage of a series of discrete discontinuities on the shaft, for example, teeth on a toothed wheel. The speed information is generally supplied to an overspeed protection system which shuts down the machine if an overspeed or runaway condition is detected. For obvious reasons, it is important in such overspeed protection systems that the sensors reliably sense speed. The ability to detect whether one or more sensors is not correctly sensing speed is also important so that the appropriate sensor(s) can be identified and, if necessary, repaired or replaced. Detection of a failed sensor may also call for automatic adjustments in the overspeed protection logic to account for such a condition.
One known method of detecting the failure of a speed sensor is to sense when the speed signal is totally lost. However, such a method is unable to distinguish between the loss of signal due to a failure of a speed sensor and the normal decay of the signal to zero which will occur during a machine coastdown to zero speed.
In another known method employed in a multiple sensor system, each speed signal is converted to an analog voltage proportional to sensed speed. If the analog voltage created by one sensor does not match the voltage created by other sensors (within certain tolerances), then the sensor with the mismatched analog voltage is considered to have failed. Such a system, however, requires complex and expensive analog detection and comparison circuitry which is beyond the capabilities of many simple applications.
It is therefore desirable to provide a method and apparatus for detecting speed sensor failure in an overspeed protection system which is simple, reliable and inexpensive to implement, while reliably distinguishing between sensor failure and normal machine coastdown. The present invention achieves these goals.